There seems to be a bug where you can build a third tower/element onto an existing 2-element tower, causing it to become invisible and stop working. It'd be nice to have some clear indication which tier-two tower you're going to create with a combination, though that's something you can trial/error+memorize
Also, I think maybe the VIP trophies should remain unavailable until prestige when you give them to the skeleton. Being able to flit through the earlier VIP levels again and just zomb-bomb the VIP and move on to get all my trophies back feels too fast/easy. Maybe that's just me.
The new skeleton champion could use a few quality of life improvements for the inventory when you have time. A 'sell all' button for one; swapping in a new item is fast and easy (one click) but the sell is a click and drag and you have to go through all of them one by one after you've optimized your loadout. Also could use a change in sort order, either flat out or as an option so you can see and compare like type. (Helmets together, swords together, etc.) And at a stretch, a bit of iconography on the tiles could indicate what bonus type they give so you can see at a glance which onces give damage vs health vs speed, etc.
This space doesn't have good threading but.. to counter BrokenAndroid's point.. by the time you can get a rune shatter, the vast majority of your zombies cost no energy at all, as they come from raising your enemies. That may not be true on very hard boss levels (relative to your prestige buys) but eventually as you said the shatters come out ahead because exponential always eventually beats linear. Even at 15 you're looking at instead of four zombies 1 that does the damage of four, but keeps going full strength after hits that would eleminate one of the smaller zombies and cut your DPS.
Very nice to see an update to this game. The skeletal champion definitely adds a bit of variation. I got a rare armor that gives random Detonate, which definitely changes things, wonder if there's other dramatic shifts like that in the cards to let you customize a bit more.
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